1. Field of the Invention
The present invention relates to lamp ballasts and, more particularly, to electronic dimming ballasts coupled to two wire phase controlled dimmers.
2. Related Art
With reference to FIG. 1, a prior art lamp system 10 includes an AC source 100 such as 120 VRMS, 60 Hz wall power, a phase controlled dimmer 102, an electronic dimmable fluorescent ballast 200, and a fluorescent lamp 300.
The ballast 200 receives input power (or hot, H) on line 202, a variable input signal (or dimmed hot, DH) on line 204, and neutral N on line 206 which is given a conventional ground symbol. It is understood that the voltages on lines 202 and 204 are rectified (for example, by full wave bridge rectifiers, not shown) within the ballast 200 to yield voltages having a positive DC average value with respect to neutral (or ground).
The electronic dimming ballast 200 is designed to provide an amount of output power to the lamp 300 in accordance with the variable input signal on line 204 from the dimmer 102. It is understood that the phase controlled dimmer 102 provides the variable input signal on line 204 by varying its phase firing angle which controls the RMS value of the variable input signal, discussed in more detail below.
As is known in the art, the ballast 200 typically includes a first power stage comprising a boost circuit 210 which receives a rectified version of the voltage on line 202 and produces a high DC voltage on line 214 which may reach 400 VDC or more.
The ballast 200 also typically includes a second power stage comprising an inverter circuit 216 (for example, a resonant converter) which converts the DC voltage on line 214 into a suitable AC voltage to drive the lamp 300. A high voltage energy storage capacitor 212 is provided in a shunt configuration with respect to line 214 to provide a low impedance source of current to the inverter 216.
The power delivered to the lamp 300 is typically provided via an output transformer 218 having a primary winding 218a and a secondary winding 218b. The transformer 218 also typically includes another secondary winding 218c, discussed below.
A control circuit 220 provides control signals and control power to the boost circuit 210 and inverter 216 over lines 221 and 222, respectively. The control circuit 220 commands the power stages (boost circuit 210 and inverter 216) to turn on or to turn off depending on certain conditions discussed below. The control signals provide information necessary to command the power stages to produce the current and voltage over line 208 which correspond with the variable voltage on line 204 such that the lamp 300 is illuminated at the proper intensity.
The control circuit 220 typically controls the inverter 216, for example, by comparing a rectified version of the variable input signal on line 204 with a signal representative of the current delivered to the lamp over line 208 and (via known error signal techniques) adjusting the control signals input to the inverter 216 over line 222 to command the proper current to the lamp 300.
As is known in the art, the control circuit 220 also commands the boost circuit 210 to produce the proper DC output voltage on line 214. Further, the control circuit 220 typically includes circuits which perform other functions such as low voltage lockout, over-current protection, over-voltage protection and the like.
The control circuit 220, boost circuit 210 and inverter circuit 216 require relatively low voltage power (or control power) to perform the conversion of the input power on line 202 to the output power on line 208. Control power is typically provided by a 15 V control circuit power supply (also known as a Vcc supply) which can deliver about 40-50 ma of current, although other voltage levels and currents may be required.
In the embodiment shown in FIG. 1, control power is provided by a control circuit power supply 240 comprising the following circuit elements: resistor 224, diode 228, low voltage storage capacitor 230, voltage regulator 232 (shown as a Zener diode), diode 229 and secondary winding 218c of the output transformer 218 of the inverter 216. It is understood that the control circuit power supply 240 may be implemented using many other circuit configurations.
The operation of the control circuit power supply 240 is now described. At start up, the lamp 300 is off and there is no output voltage on secondary winding 218c. Resistor 224, however, provides current from the input power on line 202 through diode 228 to the low voltage storage capacitor 230. The current flowing through resistor 224 to capacitor 230 produces a voltage across capacitor 230 which is sufficient to "start up" the control circuit 220 and power stages 210, 216.
The voltage regulator 232 is typically employed to ensure that the voltage across capacitor 230 does not exceed a predetermined value, for example, about 15 VDC. A Zener diode, three terminal regulator, or the like may be used for the voltage regulator 232.
The value of resistor 224 is selected such that the "trickle" current drawn from line 202 and the power dissipated in resistor 224 do not significantly affect the efficiency of the ballast 200 or overheat it. Typically, the trickle current drawn through resistor 224 does not exceed about 1-4 ma.
The current required from the control circuit power supply 240 over line 231 during normal operation of the ballast (i.e., when the power stages are substantially continuously supplying power to the lamp) is typically in the range of about 40-50 ma. The current provided through resistor 224 to the control circuit power supply 240 during start up is significantly below this level and is insufficient to operate the ballast 200 in normal operation. The amount of current provided through resistor 224 to the control circuit power supply 240, however, is high enough to charge capacitor 230 to a sufficiently high voltage to operate the boost circuit 210 and the inverter circuit 216 for a short time which enables the ballast 200 to start momentarily.
Once the inverter 216 is started, the low voltage storage capacitor 230 of the control circuit power supply 240 receives current from the secondary winding 218c of the output transformer 218 of the inverter 216 through diode 229. The turns ratio of the secondary winding 218c to the primary winding 218a is set to achieve the appropriate low voltage DC level across capacitor 230. The secondary winding 218c of the output transformer 218 provides sufficient current to the control circuit power supply 240 to operate the ballast 200 during normal operation.
The lamp system 10 of FIG. 1 has, among others, the drawback of requiring three wires between the dimmer 102 and the ballast 200, which is usually located in the light fixture itself. Consequently, the use of a fluorescent lamp dimming ballast in situations where only two wire cabling has been installed is problematic. Indeed, it is typically inconvenient or impossible to add the necessary control line 204.
One possible way to avoid the need for a three wire system is to modify the known system of FIG. 1 in the manner shown in FIG. 2. In this system the variable input signal from the dimmer 102 is connected to both lines 202 and 204 of the ballast 200. The connection between line 202 and 204 is typically provided inside the ballast 200 thus eliminating the need for a third terminal on the ballast 200 for receiving the variable input signal on line 204.
The ballast 200 of FIG. 2 operates in substantially the same way as the circuit of FIG. 1 which is advantageous in that no additional wiring is required to add dimming capability to the fluorescent lamp 300.
Although the system 10 of FIG. 2 avoids the problem of requiring three wires for dimming, it suffers from another substantial drawback because the ballast 200 may enter an oscillatory mode in which it repeatedly starts up, stops and starts up again. The above mentioned oscillatory mode occurs when the dimmer 102 is set to an insufficient phase conduction angle and, as discussed below, is encountered under two sets of circumstances.
With reference to FIG. 3, the phase conduction characteristics of the dimmer circuit 102 are now discussed. The variable input signal labeled 202a in FIG. 3 is output from a fully "on" dimmer 102 which conducts at a phase conduction angle, .phi., of about 0.degree.. The variable input signal labeled 202b is output from a dimmer 102 which conducts at some phase conduction angle, .phi., between about 0.degree. and 180.degree..
High phase conduction angles (i.e., greater than about 90.degree.) correspond with low values for the peak voltage Vp on line 202 in FIG. 2. The portions of the variable input signal labeled 202b between 0.degree. and .phi.1 and between .phi.2 and .phi.3 are called the "dead time" or "non-conduction phase periods." The portions of the variable input signal labeled 202b between .phi.1 and .phi.2 and between .phi.3 and .phi.4 are called the "conduction time" or "conduction phase periods."
The system of FIG. 2 enters the oscillatory mode when the conduction phase period (which may be measured in terms of phase angle, .phi.) or the conduction time (which may be measured in terms of time, ms) is too small. During a small phase conduction period, the peak voltage Vp on line 202 is too low to properly power the boost circuit 210, the inverter circuit 216, and/or the control circuit 220.
When the peak voltage Vp on line 202 is too low, the oscillatory mode may be triggered in two ways, namely, via over-current conditions in the boost circuit 210 or via insufficient voltage output from line 231 of the control circuit power supply 240.
Over-current triggering of the oscillatory mode is now discussed in more detail. The control circuit 220 includes an over-current protection circuit (not shown) which prevents the boost circuit 210 from drawing excessive current over line 202. It is understood that the over-current protection circuit may be disposed within the boost circuit 210 itself or another location.
When the peak voltage Vp on line 202 is too low, the boost circuit 210 may draw excessive current from line 202 in an attempt to produce the high DC voltage across capacitor 212 to power the inverter 216. This is so because the ballast 200 is designed to produce a minimum power output for the lamp 300 (i.e., just enough power to turn the lamp on) even though the dimmer 102 may be set at a high phase conduction angle (i.e., outputting a low peak voltage Vp).
Since the inverter 216 will attempt to output the minimum power level to the lamp 300 and the current drawn by the boost circuit 210 is inversely proportional to the voltage available on line 202 for a given power delivered to the lamp 300, the boost circuit 210 will draw higher currents from line 202 when the peak voltage Vp is reduced.
The higher currents drawn from line 202 will tend to trip the over-current protection circuit in the control circuit 220. By tripping the over-current protection circuit, the control circuit 220 commands the boost circuit 210 to shut down, thereby eliminating the excessive current draw by the boost circuit 210 and also shutting down the inverter 216. Thus, the filaments of the lamp 300 will have been heated (and the gas of the lamp 300 may or may not have glowed) momentarily until the boost circuit 210 reached the over-current condition.
Once the boost circuit 210 and the inverter 216 have been shut down for a sufficient period (determined by the design of the over-current protection circuit) the control circuit 220 will attempt to re-start the boost circuit 210 and the inverter 216. During the re-start, current is again drawn from line 202 and power is again delivered to the lamp 300. So long as the dimmer 102 is set at a relatively high phase conduction angle, however, the peak voltage Vp on line 202 will be too low and the boost circuit 210 will again draw excessive current. Therefore, the control circuit 220 will again shut down the boost circuit 210 and the inverter 216 and cycle power to the lamp 300.
Insufficient voltage output on line 231 from the control circuit power supply 240 may also trigger the oscillatory mode when the peak voltage Vp on line 202 is too low. The control circuit 220 includes a low voltage lockout circuit (not shown) which monitors the voltage on line 231 from the control circuit power supply 240 and shuts down the control circuit 220 (and thus the power stages) when the voltage on line 231 is too low, for example below about 10 volts.
Since the control circuit 220 and power stages draw more current from the control circuit power supply 240 after they have started, if the peak voltage Vp is too low, line 231 of the control circuit power supply 240 may not maintain a sufficiently high voltage to the control circuit 220. As a result, the voltage on line 231 of the control circuit power supply 240 may droop to the point where the low voltage lockout circuit of the control circuit 220 shuts down the power stages of the ballast 200.
After the control circuit 220 and power stages shut down, the current drawn from line 231 of the control circuit power supply 240 is reduced and the voltage on line 231 may again rise. Therefore, the low voltage lockout circuit of the control circuit 220 may again permit the power stages to start causing power to cycle in the lamp 300.
This endless cycling of power to the lamp 300 during the oscillatory mode of the ballast 200 is undesirable because the lamps are operated momentarily during each power cycle. It is well known that fluorescent lamps suffer an incremental amount of damage to their electrodes upon each start. A typical lamp will be at the end of its useful life after approximately 10,000 power cycles. Since the power cycling typically takes place at a rate of about once per second, 10,000 cycles of the lamp 300 (i.e., failure of the lamp 300) will occur after only three hours of operation in the oscillatory mode.
Even when the dimmer 102 does not fire at all during the AC line half cycles (i.e., the distance between .phi.1 and .phi.2 and the distance between .phi.3 and .phi.4 is zero degrees, the so-called "electronic off" state), the oscillatory mode of the ballast 200 can still take place. This is so because most good quality dimmers 102 contain a capacitor 104 across a semiconductor device (not shown) within the dimmer 102 to suppress RF interference. The capacitor 104 is typically of a size which allows a leakage current to flow from the AC source 100 over line 202, which leakage current is of a sufficient magnitude to charge the capacitor 230 and initiate the cycling described above. Since many dimmers now use the electronic off state instead of a switch contact (or "air-gap" off state), attempting to use a two-wire fluorescent ballast with such dimmers would again lead to very short lamp life.
Accordingly, there is a need in the art for a new ballast circuit which is capable of receiving power from a phase controlled dimmer over only two wires where the ballast will not enter an oscillatory mode when the dimmer is set to produce an output having a relatively low peak output voltage.